Monogenic lupus

DNase1L3

Homozygous loss of function mutations in DNASE1L3 were first identified by linkage analysis in a cohort of six consanguineous families with apparent Mendelian inheritance of childhood SLE. The patients in this study were notable for very early onset of disease, lack of female predominance, and high disease activity with prominent renal involvement. The affected children were further unified by the presence of ANA, anti-dsDNA antibodies, and hypocomplementemia. Most children also had anti-neutrophil cytoplasmic antibodies (ANCA). Mutations in DNASE1L3 have since been described in other patients and families with early onset SLE. In addition, homozygous frameshift deletions in DNASE1L3 have also been linked to hypocomplementemic urticarial vasculitis syndrome. These patients similarly had prepubertal onset of disease, and 3 out of the 5 children described later progressed to overt manifestations of SLE. Interestingly, lower activity levels of DNase1, a related endonuclease, and heterozygous variants in DNASE1 had been described much earlier in adults with non-familial SLE.

Multiple mechanisms may explain the link between loss of endonuclease function and development of lupus autoimmunity. Both DNase1 and DNase1L3 degrade extracellular DNA; however, DNase1L3 has an extra C-terminal peptide that facilitates its ability to digest circulating microparticle-bound DNA from apoptotic cells. This microparticle-bound DNA is capable of activating plasmacytoid and myeloid dendritic cells, resulting in the production of interferon-alpha (IFN-α). Overproduction of type 1 IFN is now recognized to be a major pro-inflammatory pathway in SLE pathogenesis. The inability to clear antigenic self-DNA may also promote expansion of autoreactive B cells and loss of B cell tolerance, resulting in the development of pathogenic anti-DNA antibodies. Mice deficient in DNase1L3 function develop early production of autoantibodies, including anti-dsDNA antibodies. Finally, there is some evidence that intracellular DNase1L3 may play a role in inflammasome activation, although it is not yet clear how this may influence lupus autoimmunity.

DNase II

DNase II is an endonuclease that, in contrast to DNase1 and DNase1L3, digests intracellular rather than extracellular DNA. Trafficking of DNase II to the lysosome is necessary for the activation of TLR9 in response to unmethylated DNA, one of the critical inflammatory responses to intracellular pathogens. DNase II also appears to be necessary for the processing of self-DNA in the chromatin of apoptotic cells and in the nuclear material expelled from erythroid precursors. In mice, deficiency of DNase II activity leads to the accumulation of undigested DNA in the phagosomes of macrophages that engulf these contents. This in turn leads to massive overproduction of IFN-β, which induces erythroid cell death and ultimately lethal anemia.

Biallelic loss of function mutations in DNASE2 were recently described in three children with lupus-like disease from two different families. All three children had profound neonatal anemia and thrombocytopenia, associated with hepatosplenomegaly and cholestatic hepatitis. These symptoms, which were later accompanied by recurrent fevers, persisted and recurred to a variable extent in childhood. One child had deforming arthritis while two had white matter abnormalities on MR imaging of the brain. All had proteinuria with features of membranous glomerulonephritis and anti-DNA antibodies. Biochemical examination of these patients demonstrated high circulating levels of type I IFN and TNF-α.

DNase III/TREX1

Mutations in TREX1 , which encodes 3’–5’ intracellular exonuclease that digests cytosolic DNA, are associated with a spectrum of lupus-like phenotypes. Familial chilblain lupus (FCL) is characterized by early onset of acral, cold-induced vasculitic skin lesions. Histopathology of these lesions is similar to that seen in lupus. Patients additionally have variable presence of arthralgias and autoantibodies, but otherwise do not show manifestations of systemic disease. Inheritance is autosomal dominant, associated with heterozygous loss-of-function mutations in TREX1. Biallelic mutations and certain heterozygous mutations are also linked to a more severe phenotype, Aicardi-Goutieres syndrome (AGS). Children with AGS develop severe neurologic disease, with neonatal encephalitis, intracranial calcifications, and progressive neurologic decline. High levels of IFN-α in the cerebrospinal fluid have been reported. Some patients demonstrate vasculitic skin lesions similar to those seen in FCL. The link to lupus is further strengthened by the finding that TREX1 polymorphisms are also found in higher frequency in cohorts of non-Mendelian SLE, and may be associated with neurologic manifestations in particular. Finally, true monogenic lupus due to TREX1 mutation was described in one case of a 4 year old child with SLE and cerebral vasculitis.

The TREX1 exonuclease, also known as DNase III, is speculated to play a role in excision repair after DNA damage, but mice deficient in TREX1 do not show increased rates of cancer or gene mutations. Rather, they develop overwhelming myocarditis, suggesting that the primary role of TREX1 may be more to inhibit inflammatory responses. The nucleic acid target most responsible for provoking inflammation in the setting of TREX1 deficiency remains unclear; single-stranded self DNA and RNA as well as endogenous retroelements have all been proposed. In one hypothesis, oxidized DNA from pathogens or from UV exposure (as is known to potentiate lupus activity) is more resistant to TREX1 degradation and thus more immunogenic.

Other genes specifically linked to AGS include RNASEH2A , RNASEH2B , RNASEH2C , SAMHD1 , and ADAR1. Like TREX1 , the products encoded by these genes are all related to processing of nucleic acid. RNASEH2A , RNASEH2B , and RNASEH2C encode the enzymes that together form the RNase H2 complex, a nuclease, which digests RNA:DNA hybrids formed during genomic replication. Although the mechanism by which deficiency of RNase H2 function leads to AGS is not entirely clear, evidence suggests that consequent instability of the genome during replication leads to accumulation of cytosolic double-stranded DNA as seen in TREX1 deficiency. SAMHD1 is a triphosphohydrolase enzyme which metabolizes dNTPs and is itself an IFN-inducible protein. The enzyme is thought to be important for the antiviral response, but mutations in SAMHD1 are also linked to carcinogenesis. More recent studies have demonstrated that SAMHD1 plays an important role in DNA replication by activating other exonucleases at stalled replication forks. Deficiency of SAMHD1 function allows the cytosolic accumulation of single-stranded DNA (ssDNA), again stimulating the production of type 1 IFN. Adenosine deaminase acting on RNA (ADAR1) catalyzes the deamination of adenosine to inosine during RNA editing. Biallelic mutations in ADAR1 were reported in a cohort of AGS families, and more recently heterozygous mutation in ADAR1 has been linked to Evans syndrome (autoimmune hemolytic anemia and immune thrombocytopenia). Interestingly, prior to the link to AGS, heterozygous mutations in ADAR1 had been linked to dyschromatosis symmetrica hereditaria, an autosomal dominant pigmentation disorder. The range of mutations and phenotypes appears to be broad, with neurologic manifestations and chilblain lesions reminiscent of AGS reported in some patients.